Pregnant women represent only about 1 percent of the population of the United States, yet they accounted for 5 percent of deaths from the H1N1 flu between April and August of 2009, according to an analysis of data from the Centers of Disease Control in Atlanta, Georgia, by Alicia M. Siston, Ph.D., of the CDC, and colleagues.

Taking antiviral drugs soon after they became ill greatly helped pregnant women who were hospitalized.

Of 788 pregnant women who were reported to the CDC to have become ill with H1N1 virus between April and August, 30 died. That was 5 percent of all swine flu deaths for the period. Of 509 women who were hospitalized, 115 were so sick they were admitted to intensive care units.

If they had waited four days after the onset of symptoms to go to a doctor, pregnant women were six times more likely to wind up in an ICU than if they sought treatment after only two days.

Pregnant women should be vaccinated against H1N1, and should be treated quickly with antiviral drugs if they do become sick, the authors recommended.

Two-thirds of the women who died in the final tally for the year were in their final trimester of pregnancy. “Changes in the immune, cardiac, and respiratory systems are likely reasons that pregnant women are at increased risk for severe illness with influenza,” the authors wrote.